Linear motor, a linear compressor, a method of controlling a linear compressor, a cooling system, and a linear compressor controlling a system

ABSTRACT

A linear compressor ( 100 ) applicable to a cooling system ( 20 ) includes a piston ( 1 ) driven by a linear motor ( 10 ), the piston ( 1 ) having displacement range controlled by means of a controlled voltage (V M ), the controlled voltage (V M ) having a voltage frequency (φ P ) applied to the linear motor ( 10 ) and adjusted by a processing unit ( 22 ), the range of piston ( 1 ) displacement being dynamically controlled in function of a variable demand of the cooling system ( 20 ), the linear compressor ( 100 ) having a resonance frequency, the processing unit ( 22 ) adjusting the range of piston ( 1 ) displacement, so that the linear compressor ( 100 ) will be dynamically kept on resonance throughout the variations in demand of the cooling system ( 20 ).

This application is a divisional of U.S. application Ser. No. 10/587,234filed Oct. 30, 2006, now U.S. Pat. No. 8,033,795 which is the U.S.National Phase filing of PCT International Application No.PCT/BR2005/000006 having an international filing date of Jan. 19, 2005,which claims priority from Brazilian patent application No PI0400108-7filed on Jan. 22, 2004, all the disclosures thereof being herebyincorporated herein by reference.

The present invention relates to a linear motor, a linear compressor, alinear compressor controlling method, a cooling system, as well as to alinear compressor controlling system with the purpose of operating alinear compressor in resonance, so that the latter will have the highestefficiency throughout its operation.

BACKGROUND OF THE INVENTION

A resonant linear motor essentially comprises a linear motor, forexample, a linear motor coupled to a resonant mechanism, which maycomprise a spring, or to a to any load that produces a spring effect, togenerate a resonant movement between the linear motor and the load. Theapplications of this type of linear motor may include driving fluidpumps in general, which can actuate variable loads.

Typical examples of this type of construction are linear motors employedon linear compressors that are often applied to cooling systems, due toits efficiency in terms of economy of electric energy. A linearcompressor 100 employed on a cooling system is, as shown in FIG. 1,usually mounted inside a housing (not shown), the gas contained in thishousing being under low pressure and being aspirated and compressed bythe linear compressor for release in a high-pressure environment 7.

The gas compression mechanism takes place by axial movement of thepiston 1 inside a cylinder 2 having a head 3; suction 3 a and discharge3 b valves being positioned at the head 3, these valves regulating theentry and exit of gas in and out of the cylinder 2. The piston 1 isdriven by a linear motor 10, which is formed by a stator 411 having acoil 11 and a support 4. The stator 411, in turn, actuates a magnet 5that impels the actuator, in this case the piston 1, the actuator beingassociated to a helical-type spring 8, forming the resonant assembly ofthe linear compressor 100.

The resonant assembly, driven by the linear motor 10 that has thefunction of producing a linear reciprocating movement, causing themovement of the piston 1 inside the cylinder 2 to exert a compressionaction compressing the gas admitted by the suction valve 3 a, to theextent where the latter can be discharged to the high-pressure sidethrough the discharge valve 3 b into the tubing 7.

The amplitude of the operation of the linear compressor 100 is regulatedwith the balance of the power generated by the linear motor 10 and thepower consumed by the mechanism in compressing the gas plus otherlosses.

Another characteristic of the linear mechanism is the possibility ofvarying its pumping capacity, reducing the power of the electric motor,the operation amplitude in turn reducing the pumping capacity.

A parameter to be varied for controlling the amplitude of the linearcompressor may be the feed voltage of the electric motor. From the feedvoltage of the electric motor until the desired amplitude is achievedthere are various coupled impedances, such as electric resistance of theelectric motor, the inductance of the electric motor, capacitance if acapacitor is used, the counter-electromotive force, the impedances ofthe resonant system (mass/spring) and the compression work with itsinherent losses. The impedance of this system depends upon the actuationfrequency of the system, that is to say, the frequency of the voltageapplied to the electric motor. At a certain frequency the output of thissystem is optimum, and this occurs when the mechanical system entersinto resonance; at this frequency the output of the linear compressor ismaximum.

“Gas Staring” Effect

The resonance frequency of the mechanism is not perfectly fixed. Whencompressed, the gas has a mechanical effect similar to the one of aspring (also known as “gas spring”), this “gas spring” is affectedmainly by two factors: the distance between the piston and the valveplate and the pressures that the linear compressor operates.

The distance between the piston and the plate is altered when the pistonstroke is reduced, generating an increase of the “gas spring” and in theresonance of the mechanism (this effect is the most relevant to theoperation stability of the mechanism). In a cooling system, these twofactors alter substantially, the pressures varying from the instant whenthe system is turned on until it reaches the operation condition, theoperation condition is affected by the room temperature and the internaltemperature of the cooler, the piston/plate distance is altered when thesystem needs more or less energy for its operation. In this way, theresonance frequency of the mechanical system varies due to variousfactors.

Cooling System/Cooler/refrigerators Usable with the Teachings of thePresent Invention

There are basically two types of cooler: the simple-class coolers andthe coolers with embarked electronics. In addition to the application tocoolers in general, the teachings of the present invention may beapplied to cooling systems in general, for instance, air conditioningsystems. In this case, the only conceptual difference lies in the factthat the air conditioning system is applied to a room (or cooledenvironment), whereas in the case of a cooler or a refrigerator thesystem is used in a closed cabinet.

Anyway, the coolers or cooling systems with embarked electronics areprovided with electronic circuits that have the capacity of analyzingthe internal temperature of the cooler and making adjustments in thecapacity of the linear compressor so as to operate it in the mosteffective way possible.

The coolers or cooling systems of this simple class are not providedwith embarked electronics, having only one circuit that turns on and offthe linear compressor (an “on/off” thermostat) from time to timewithout, however, being able to act on the capacity thereof.

In spite of operating in an efficient way, the coolers with embarkedelectronics obviously have a higher cost when compared with thesimple-class coolers.

According to the teachings of the present invention, it is possible toprovide a linear compressor with electronics capabilities to adjust therespective capacity, following the demand of the cooling system, even inthe cases where simple-class cooler are employed. For this purpose, thelinear compressor should be capable of analyzing the cooling capacitynecessary for the required condition within the environment of a cooler,based on measurements made in the feed voltage and current of theelectric motor that drives the linear compressor.

DESCRIPTION OF THE PRIOR ART

One of the ways of achieving an improved efficiency in systems involvinglinear compressors is to approximate the piston to the respective strokeend as far as possible. Examples of these techniques may be found indocuments U.S. Pat. No. 5,156,005 and U.S. Pat. No. 5,342,176. In thesetwo documents, the control over piston range is described. Neither ofthese techniques, however, foresees control over the amplitude for thelinear compressor to operate in resonance, so that, on the basis of theteachings of these documents, the linear compressor may operate with lowefficiency depending upon its load conditions.

A prior art which describes a system that monitors the movement of thepiston of a linear compressor, is disclosed in document WO 01/54253.According to the teachings of this document, a system and a methodapplicable to a linear compressor are foreseen, according to which themeasurement of a first square wave obtained by integrating the currentapplied to the electric motor and a second square wave obtained from thevoltage applied to the electric motor. On the basis of thesemeasurements, the control over the movement of the piston is carried outby means of a TRIAC, evaluating the difference between the phases of thefirst square wave and the second square wave. In this way, the piston ofthe linear compressor operates in a position closer to the valve plate.

This document WO 01/54253 does not approach the problem of unbalancerelating to the “gas spring” effect, and one of the objectives of theinvention described in this prior art document is to obtain a greateroperation stability, and so the system can operate in non-idealconditions in terms of efficiency.

Moreover, another drawback resulting from the construction proposed indocument WO 01/54253 lies in the fact that one monitors the phase bymeans of square waves. This approach results in that, to obtain thedifference between the phases, it is necessary to employ electroniccircuits or computer programs that make the integration of the current,that generate the first and the second square waves described thereinand that calculate the difference in phase between the first wave andthe second square wave. This construction, and the process describedtherein, however, has a high manufacture cost in addition to lessreliability, since it needs circuits for carrying out these conversions,which, due to the large number of components involved, reduces thereliability of the system, since each added electronic componentrepresents a greater probability of failure. The option of implementinga device described in the document by means of a computer program alsoresults in a high cost, since in this case, with the approach used, itwill be necessary to have an excessively sophisticated microcontrollerand, therefore, higher costs.

Moreover, the application of linear compressors in cooling systemsdepends on the use of electronic thermostats capable of informing thecontrol, by means of a command signal, about the capacity which thelinear compressor should operate. This makes the system complex and doesnot permit application of the linear compressor in any system.

Another possibility is to actuate the linear compressor in a fixedcapacity and to use the conventional thermostat (“On/Off” type). This,however, sub-utilizes the resources available at the control.

OBJECTIVES OF THE INVENTION

The objectives of the present invention are a linear motor, a linearcompressor, a method of controlling it, as well as a cooler/refrigeratorthat has no need of embarked electronics, but at the same time has thecapacity of adjusting the capacity according to the demand. In otherwords, according to the teachings of the present invention, the coolerwill see the linear compressor with electronics as if the latter was anordinary linear compressor, thus maintaining the characteristics ofsimple-class coolers/refrigerators unchanged and transferring the wholeelectronic control to the linear compressor.

In addition, it is an objective of the present invention to provide aresonant linear motor that can operate in a controlled way as far as therange of its displacement is concerned, without the need to make use ofan external electronic control.

Thus, the intention is:

-   -   To adjust the operation capacity of the cooling system by using        a linear compressor, without the need for a complex thermostat,        permitting the use of the linear compressor in any type of        system.    -   To permit the use of the linear compressor in any system with a        conventional thermostat (on/off type), and further permit the        adjustment of the operation capacity, thus using the whole        potential of the linear compressor.    -   To optimize the functioning of the linear compressor, to have        the system always operate at its maximum efficiency possible.    -   To operate a resonant linear motor, without the need to use an        external circuit controlling its behavior; the latter should        operate always in optimum functioning conditions.

SUMMARY OF THE INVENTION

As described above, the resonance frequency of the mechanism varies as afunction of the pressures and of the operation amplitude of the linercompressor. Since the pressures are variable and non-controllable (atleast directly) at certain moments, the linear compressor may operateout of resonance, which results in loss of performance. On the otherhand, the displacement range or piston stroke is a controllablevariable, so that according to the present invention it is foreseen tovary/adjust the operation stroke so as to minimize or zero the phasebetween the current and the piston speed, so as to maintain themechanism in perfect syntony and consequently with the best performance.Analyzing a cooling system, one can notice that the aspiration pressureof the linear compressor rises when the door of the cooler is opened ora new thermal load is added to the system. In this situation, thestrategy of maintaining the phase minimized by varying the operationamplitude causes the linear compressor to increase the stroke, thusmeeting the necessity of the system to withdraw the added heat.

It is important to observe that, in spite of making reference to thereading of the phase between the current and the piston speed, oneshould understand that this phase may be obtained by reading otherparameters; for instance, one may replace the piston speed by the pistondisplacement, which is at 90° with respect to each other, and may usethis information to read the phase by using the piston position as areference, for some constructive convenience of the control circuit. Itis also possible to replace the speed by the counter-electromotive force(CEMF) that is in phase, with the objective to measure the phase betweenthe current and the dynamics of the mechanism (for example, the CEMF).Preferably, the mean of current phase and the CEMF phase are used,resulting in the electric-motor phase.

The CEMF may be obtained by the formula

${CEMF} = {k \times \frac{\partial D_{P}}{\partial t}}$wherein k is a constant; δD_(P) is the piston-displacement derivative,and δt is the time derivative.

Since the piston movement is approximately sinusoidal it is possible toknow when the displacement is maximum, the CEMF is passing by zero. Inorder to detect this value, it is sufficient to have a proximity sensor,which will indicate a peak of signal when the piston is, for instance,close to its stroke end. Thus, in order to measure the CEMF phase, thepoint of maximum piston displacement is measured.

With regard to the behavior of the resonance, it is known that, as therange of piston displacement is increased, the resonance frequency ofthe linear compressor is lowered, while the greater the load demanded bythe cooling system the higher the resonance frequency.

This phenomenon occurs because, as the suction pressure increases(demanded by the cooling system at the suction valve of the linearcompressor), this means that a thermal load has been put into thecooler. This warmer mass raises the temperature of the internalenvironment of the cooling system, causing the rise in the evaporationpressure, since the cooling liquid is in a saturated liquid state, andone can conclude that the pressure and the temperature are intrinsicallyinterconnected. Thus, the fact of placing something warmer in thecooler, will result in a rise in pressure, raising the pressure of thegas on the piston, causing the resonance of the mechanism to decrease,which will cause a phase shift in the linear compressor.

In practice, this means that, as the heat inside the cooler rises, theload of the system also rises, causing the resonance frequency of thesystem to rise, and the piston stroke should be increased, which resultsin reducing of resonance frequency, since the piston stroke is longer.In this case, that difference by which the resonance frequency has risenas a function of the load added to the system, may cause the system tooperate again at the previous frequency (or resonance frequency),increasing the piston displacement, leading the assembly to operate inresonance frequency.

On the other hand, a decrease in the load of the system (frozen food,lowering the ambient temperature) leads to an increase in the systemphase, which may render the phase positive and compensate with adecrease in the system capacity until the phase reaches the zero value.

In this way, while the phase shift is positive, one should decrease thecooler capacity, to have the system operating again in resonance and,when the phase shift is negative, the capacity of the linear compressorshould be increased, to have the system operating again in resonance.

One of the objectives of the present invention is achieved by means of alinear motor having a displacement range that is controlled by means ofthe electric voltage controlled by the processing unit, so that theresonant assembly can be dynamically kept in resonance throughout thevariations in load.

Further, one of the objectives of the present invention is achieved bymeans of a linear compressor applicable to a cooling system, the linearcompressor comprising a piston driven by an electric motor, the pistonhaving a displacement range controlled by a controlled electric voltage,the controlled electric voltage having a voltage frequency applied tothe electric motor and adjusted by a processing unit, the range ofpiston displacement is dynamically controlled as a function of thevariable demand of the cooling system, the linear compressor having aresonance frequency, the processing unit adjusting the range of pistondisplacement so that the linear compressor will be dynamically kept inresonance throughout the variations in demand of the cooling system.

The objectives of the present invention are also achieved by means of amethod of controlling a linear compressor, the linear compressorcomprising a piston driven by an electric motor, the piston having adisplacement range controlled by a controlled electric voltage, thecontrolled electric voltage having a voltage frequency applied to theelectric motor and adjusted by a processing unit, the method comprisingthe steps of monitoring the range of piston displacement throughout theoperation of the linear compressor, dynamically adjusting thedisplacement range as a function of a variation in the demand of thelinear compressor, so that the linear compressor will be kept inresonance throughout the variations in demand of the cooling system.

Further, the objectives of the present invention are achieved by meansof a method of controlling a linear compressor, which comprises steps ofmeasuring a feed phase of the feed current and a dynamic phase of thepiston of the linear compressor, and measuring and establishing ameasured phase, and dynamically adjusting the displacement range as afunction of a variation in the demand of the linear compressor, so thatthe linear compressor will be kept in resonance throughout thevariations in the demand of the cooling system.

The teachings of the present invention are further carried out by meansof a cooling system comprising a linear compressor, the cooling systemcomprising an on/off thermostat actuating the linear compressor, thelinear compressor comprising a piston driven by an electric motor, thepiston having a displacement range controlled by a controlled electricvoltage, the controlled electric voltage having a voltage frequencyapplied to the electric motor and adjusted by a processing unit, therange of piston displacement being dynamically controlled as a functionof a variable demand of the cooling system during the period in whichthe thermostat turns on the linear compressor, the linear compressorhaving a resonance frequency, the processing unit adjusting the range ofpiston displacement, so that the linear compressor will be dynamicallydept in resonance throughout the variations in demand of the coolingsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in greater detail withreference to embodiments represented in the drawings. The figures show:

FIG. 1 is a schematic view of a linear compressor;

FIG. 2 is a diagram of the control system, of the linear compressor andof the cooling system of the present invention;

FIG. 3 is a block diagram of the control system, the linear compressorand the cooling system, illustrating the use with a conventionalthermostat;

FIG. 4 is a block diagram of the control system of the presentinvention;

FIG. 5 is a block diagram of the algorithm of automatic control of thecapacity applicable to the linear compressor and the cooling system ofthe present invention;

FIG. 6 represents a curve of the load of the electric motor in functionof the phase;

FIG. 7 represents a curve of the capacity of the electric motor infunction of the phase for various loads;

FIG. 8 is a time diagram illustrating the wave forms of the voltagenetwork, counter-electromotive force (CEMF), current of the electricmotor, position of the piston and signal of the sensor in the situationin which the linear compressor is operating at the resonance(φ_(PC)=φ_(P)−φ_(C)=0);

FIG. 9 is a time diagram illustrating the wave forms of the voltagenetwork, counter-electromotive force (CEMF), current of the electricmotor, position of the piston and signal of the sensor in the situationin which the linear compressor is operating above the resonance(φ_(PC)=φ_(P)−φ_(C)>0);

FIG. 10 is a time diagram illustrating the wave forms of the networkvoltage, counter-electromotive force (CEMF), current of the electricmotor, position of the piston and signal of the sensor in the situationin which the linear compressor is operating below the resonance(φ_(PC)=φ_(P)−φ_(C)<0);

FIG. 11 is a flow diagram of the method for controlling the linearcompressor of the present invention;

FIG. 12 represents a curve of the load of the electric motor in functionof the phase, when the teachings of the present invention are employed,according to a second preferred embodiment;

FIG. 13 represents a curve of the capacity of the electric motor infunction of the phase for various loads when the teachings of thepresent invention are employed, according to a second preferredembodiment; and

FIG. 14 represents a flow diagram of the method for controlling thelinear compressor, according to a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE FIGURES

As can be seen, FIG. 2 illustrates a system comprising acooler/refrigerator with the embarked electronics. In this case, anelectronic thermostat 25 is integrated into the cooling system 20 andsupplies a reference signal for the processing unit 22. The processingunit 22, in turn, controls the linear compressor 100, receiving a signalfrom the sensor SS corresponding to the displacement of the piston 1.

Electronic Controls

FIG. 3 illustrates a cooling system 20 applied to a simple-class cooler.As can be seen, in this case, the cooling system 20 supplies only asignal that turn the processing unit 22 on and off. In this way, thecooling system 20 can dispense all the electronics foreseen in thecoolers/cooling systems that comprise embarked electronics. Moreover,with this construction the processing unit 22 can be integrated to thelinear compressor 100 (see indication 31), which may be supplied forvarious coolers/refrigerator/cooling systems 20 manufacturers, resultingin an equipment with high flexibility when compared with pieces ofequipment of the prior art. A proximity sensor 30 associated to theprocessing unit 22 will provide the position of the piston 1 when thelatter comes close to the respective stroke end. In practice, minorvariations in the piston stroke correspond to great variations in thecapacity of the linear compressor, so that, by way of example, for amaximum stroke of 8.5 mm (maximum capacity) the minimum stroke would beabout 6.5 mm (capacity close to zero), that is, about 2 mm of strokevariation range for the capacity to vary from zero to the maximum.

FIG. 4 illustrates a detail of the processing unit 22. As can be seen,the processing unit 22 comprises a microcontroller 40, which controls aTRIAC 41 through a gate 42. The microcontroller 40 receives the signalsof detection of zero of the network V_(AC) voltage ZT, as well as thezero signal of the current ZC at the exit of the TRIAC 41. Adisplacement reference signal REF may be supplied by the cooler, if acooler with embarked electronics is used. The main signal for thepresent invention refers to the displacement signal DP, which isobtained from the signal SS of the proximity sensor 30 and that shouldbe treated, for instance, according to the teachings of Brazilian patentdocument PI 0301969-1, the description of which is incorporated hereinby reference. Optionally, one may use the value of the feed currenti_(A) at a moment different from ZC; for this purpose, one should makethe necessary adjustments in order to have the correct measurements.

Control Algorithm

FIG. 5 illustrates the algorithm according to a first embodiment of thepresent invention for obtaining the controlled voltage V_(M) that shouldbe applied to the linear motor 10, so that one can keep the linearcompressor 100 in resonance. As can be seen, in order to calculate thevalue of the controlled voltage V_(M), it is necessary to calculate ameasured phase φ_(PC), which is obtained from the difference between thedynamic phase φ_(P) and the current feed phase φ_(C):φ_(PC)=φ_(P)−φ_(C)

The calculation of the current phase or feed phase φ_(C) is made fromthe zero of the current ZC and from the zero voltage zero ZT, whereasthe calculation of the piston-displacement phase or dynamic phase φ_(P)is made from the piston-displacement signal DP and from the zero of thevoltage ZT.

Further with respect to the obtention of the feed phase φ_(C), the feedcurrent i_(A) may not have a passage by zero, which would make itpossible to capture a pre-defined moment for establishing the zero ofthe current ZC. This may be observed, for instance, in FIGS. 8, 9 and10, where the feed current i_(A) remains at zero for a certain period.In this case, one should consider the pre-defined moment as the averagepoint of permanence of the feed current i_(A) at zero.

From the values of the dynamic phase φ_(P) and the feed phase φ_(C), onecan obtain the value of the measured phase φ_(PC) and obtain thereference value of the maximum piston displacement DP_(REF)(displacement that one expects to achieve a defined physical position).This value may be obtained by means of the algorithm of FIG. 11.

Once the reference value of maximum piston displacement DP_(REF) hasbeen obtained, it is sufficient to subtract from it the maximum pistondisplacement DP_(MAX) by the equation:E _(DP) =DP _(REF) −DP _(MAX)

to obtain the error value E_(DP) between reference maximum pistondisplacement DP_(REF) and the maximum of piston displacement DP_(MAX).

From this result it is possible to obtain the value of a control voltageV_(P), since its value is a function of the error E_(DP). Thisrelationship may be observed in the flow diagram in FIG. 11. Therein onemay change increasing capacity with increasing DP_(REF), and may changedecreasing capacity with decreasing DP_(REF). Alternatively, one mayalso use, for instance, a traditional method such as a PID algorithm toalter DP_(REF); in this case, the calculation would be made from thefollowing equation:

${DP}_{REF} = {{K_{P} \times \phi_{PC}} + {K_{D} \times \left( \frac{\partial\phi_{PC}}{\partial T} \right)} + {{Ki} \times {\int{\phi_{PC} \times {\partial T}}}}}$wherein K_(P) is a proportional constant, KD is a derived constant andKi is an integral constant, as known from the classic nomenclature incontrol.

Further one may directly increase or decrease the value of the controlvoltage V_(P), since this value if a function of φ_(PC). In this case,in the flow chart of FIG. 11, one may change increasing capacity withincreasing V_(P), and may change decreasing capacity with decreasingV_(P), so that in this option one may also use some traditional methodsuch as PID algorithm to alter V_(P) from φ_(PC) by using the followingequation:

$V_{P} = {{K_{P} \times \phi_{PC}} + {K_{D} \times \left( \frac{\partial\phi_{PC}}{\partial t} \right)} + {{Ki} \times {\int{\phi_{PC} \times {\partial t}}}}}$

The constants are the same as described before.

From the value of the control voltage V_(P) it is possible to adjust thecontrolled voltage V_(M) by calculating the trigger angle of the TRIAC.

According to the graph of FIGS. 6 and 7, increases in the system load(increase in room temperature, increase in the thermal load in thesystem) lead to a decrease in the system phase. If this increase in theload is large (see dashed line with indication of “maximum load” in FIG.7) the phase will go on to negative values; this can be compensated byan increase in the capacity of the system (increase in the piston stroke1), which will increase the phase, so that successive increments in thecapacity lead the phase to the zero value, that is to say, the systemwill be operating in resonance. In an equivalent way, a decrease in theload (see dashed line with indication of “minimum load” in FIG. 7) thephase will go to positive values, and this variation can be compensatedby an Increase in phase, so that successive Increments lead the value ofthe phase to zero, that is to say, the system will be operating inresonance.

As far as the manner of making the increase and the decrease in phase isconcerned, the reading of the feed phase φ_(C) and the dynamic phaseφ_(P) every cycle or semi-cycle should be foreseen. So, whenever themeasured phase φ_(PC) is different from zero, the control system shouldactuate on the piston displacement, and the reading of the dynamic phaseφ_(PC) may be made according to the teachings of Brazilian patentdocument PI 0300010-9, which is incorporated herein by reference.

The amplitude of the decrements should take into consideration thereaction of the system in response to the increment/decrement caused bythe control system. Thus, if the value of the increment/decrement ishigh, a longer stabilization time will be required; in the contrarycase, the stabilization time will be shorter. Typically, thestabilization time depends upon the constants of the compressor time andof the cooling system. By way of example, one may opt for awaiting apredetermined time, for instance about 10 to 60 seconds, or monitor thesystem phase until the latter remains constant.

Optionally, it is possible to use variable increment/decrement values.In this case, if the measured phase φ_(PC) is large, one may use largerincrements/decrements, and decrease this value as the value of themeasured phase φ_(PC) comes close to zero. In this case, one may opt fora reference value of 1% of increment/decrement.

FIG. 8 shows a time diagram illustrating the wave forms of voltage ofthe network V_(AC) of the counter-electromotive force (CEMF), of thecurrent i_(A) of the linear motor 10, of the piston position DP and ofthe signal of the proximity sensor (not shown) in the situation wherethe linear compressor 100 is operating in resonance, that is to say,when φ_(PC)=φ_(P)−φ_(C)=0.

As can be seen, in the situation of resonance, the piston displacementis maximum when the feed current i_(A) of the linear motor 10 passes byzero, a moment when the proximity sensor shows a measurable signal (seeindication 80). In this condition, linear compressor 100 operates inoptimum condition, since in this case the feed current i_(A) passes byzero at the moment when the piston 1 is changing direction in its path,that is to say, it passes by a moment of maximum displacement, whenthere is no need for application of force onto it, since when the piston1 is at mid-displacement (see indication 82) the feed current i_(A) andthe CEMF are maximum, impelling the piston 1 in the most effective waypossible.

In FIG. 9 one can observe that the linear compressor 100 is operatingabove the resonance, that is to say, the CEMF is in delay with respectto the feed current i_(A) of the linear motor 10. In this case, theequation is φ_(PC)=φ_(P)−φ_(C)>0, and one should increment the capacityof the linear compressor 100 by raising the controlled voltage V_(M). Itcan be noted that, in this situation, when the piston 1 is at maximumdisplacement of its path, a moment when no feed current i_(A) should beapplied to the linear motor 10, the feed current i_(A) already has asignificant value at this moment. According to the same situation ofphase shift, at the moment when the piston 1 is at the middle of itspath (see indication 90), a moment when the maximum feed current i_(A)should be applied to the linear motor 10, the feed current i_(A) hasalready undergone a decrease in its level, so that in the two situationsthere is a waste of energy and, therefore, a reduced efficiency in theoperation of the linear compressor 100 as a whole.

In FIG. 10 one can observe that the linear compressor is operating belowthe resonance; in this case the CEMF is advanced with respect to thefeed current i_(A) of the linear motor 10, and the equation is thenφ_(PC)=φ_(P)−φ_(C)<0. In this case, one should increment the capacity ofthe linear compressor 100, to have the system operating in resonance.

As can be seen, in this situation there is a delay in the phases, whichcauses the linear compressor to operate with low efficacy, since at themoment when the piston displacement is maximum, a situation on which nofeed current i_(A) should be applied to the linear motor 10, one canobserve that the feed current i_(A) is not null. Moreover, at the momentwhen the piston 1 is at the middle of the displacement (see indication101), a moment when a maximum of feed current i_(A) should be applied tothe linear motor 10, the feed current i_(A) is not maximum, so that, inthis case too, the linear compressor 100 has its efficiency reduced.

Application in Linear Compressors

Structurally, the linear compressor 100 and the system of controlling alinear compressor 100 have the following characteristics:

The linear compressor 100 comprises a piston 1 and is driven by thelinear motor 10, which brings about a displacement range that will becontrolled through the controlled voltage V_(M), this controlled voltageV_(M) having a voltage frequency f_(p). The range of piston 1displacement is dynamically controlled as a function of the variabledemand of the cooling system 20, through the processing unit 22, whichadjusts the range of piston displacement, so that the linear compressor100 will be dynamically kept in resonance throughout the variations indemand of the cooling system 20, that is to say, so that itsdisplacement range will be adjusted throughout the changes resultingfrom the variations in load demanded by the cooling system 20, impellingthe linear compressor to operate in resonance. The system of controllingthe linear compressor 100, when taken in isolation, should be applicableto the linear compressor so as to make the dynamic adjustment of thedisplacement range, to have the linear compressor operating inresonance.

Application in Cooling Systems

The cooling system 20, which may include a cooler/refrigerator or anair-conditioning system and analogous systems, as already commented,should comprise an on/off thermostat actuated by the linear compressor100, to have the range of piston displacement dynamically controlled asa function of the variable demand of the cooling system 20 during theperiod when the thermostat turns on the linear compressor. Theprocessing unit 22 should dynamically adjust the range of pistondisplacement, so to keep the linear compressor in resonance throughoutthe variations of demand of the cooling system 20.

In order to control the linear compressor 100, the control system andthe cooling system 20 of the present invention are provided with amethod of controlling the linear compressor 100 that follows the flowchart illustrated in FIG. 11.

The control over the range of piston 1 displacement is made by means ofa controlled voltage V_(M), which is adjusted by the processing unit 22.In order to adjust the level of the controlled voltage V_(M), one mayopt for following the teachings of a brazilin patent document PI9907432-0, which is incorporated herein by reference.

Application in Linear Motors

Bearing in mind that, with a control usable on linear compressors ingeneral, one can make use of the teachings of the present invention on alinear motor 10 applied to other types of utilization. In this case, anactuator (not shown) has the same function of the piston 1 used in thecompressor 100, that is to say, the actuator receives the forcegenerated at the stator 411, moving the load and forming a resonantassembly that will have a resonance frequency.

In an analogous way as foreseen for the control over the linearcompressor 100, the actuator has a displacement range that will becontrolled by means of the controlled voltage V_(M) from the processingunit 22, so that the resonant assembly will be dynamically kept inresonance throughout the variations of load.

The control over the linear motor 10 may also be made by means of theprocessing unit 22, which measures the feed phase φ_(C) of the feedcurrent i_(A) and of the dynamic phase φ_(P), in this case of theactuator rather than of the piston, and adjusts the controlled voltageV_(M), so that the value of the measured phase φ_(PC) will be null.

Also, one may control the linear motor 10 by using a variable frequencyinverter, which should be dynamically adjusted to the voltage frequencyf_(VM) of the controlled voltage V_(M) to a value equal to the value ofthe resonance frequency of the resonant assembly, as the load variationsoccur.

Control Method by Phase Adjustment

In order to carry out the control method, the processing unit 22monitors the range of piston 1 displacement throughout the operation ofthe linear compressor 100 and dynamically adjusts the displacement rangeas a function of a variation in demand of the linear compressor 100, sothat the linear compressor 100 will be kept in resonance throughout thevariations in demand of the cooling system 20.

In order to impel the linear compressor 100 to operate in resonance, onemeasures the feed phase φ_(C) of a feed current i_(A) and the dynamicphase φ_(P) of the piston 1 of the linear compressor 100 and measuresthe difference between the measured phases to establish the measuredphase φ_(PC).

After the step of establishing the measured phase φ_(PC), one shouldincrement the range of piston 1 displacement when the value of themeasured phase φ_(PC) is positive or a step of decreasing the range ofpiston 1 displacement when the value of the measured phase φ_(PC) isnegative, and one should always increase or decrease the displacement ofthe piston 1 to a value necessary for the measured phase φ_(PC) to benull.

By preference, after the step of increasing or decreasing the range ofpiston 1 displacement, one should wait until a stabilization time haspassed before measuring again the difference between the feed phaseφ_(C) and the dynamic phase φ_(P).

Control Method by Adjustment of Phase Frequency

According to a second preferred embodiment of the present invention,another way of controlling the compressor to control the frequencyapplied to the motor to keep it will always operating in resonance.

In this case, the control is made through the variable frequency, byusing a variable frequency inverter (not shown). In this way, when theload applied to the linear compressor 100 is changed, there will also bea change on the dynamic phase φ_(P) of the system, which will bedetected by the control system of the present invention, in order toalter the frequency for the compressor to operate in resonance. Thiscontrol is dynamically made by adjusting the voltage frequency f_(VP),through the variable frequency inverter, to a value equal to theresonance frequency of the linear compressor 100, as the variations indemand of the cooling system 20 occur.

The ways of making this kind of adjustment may include, for example,varying the frequency so as to minimize the feed current or else varyingthe frequency so as to zero the phase between current and the CEMF.

As can be seen in FIGS. 12 and 13, when the load increases, thefrequency of the linear compressor 100 increases, and one shouldincrement the respective capacity, to have the system operating inresonance and vice-versa, when the load decreases, that is to say, thesystem should increase the piston 1 stroke/capacity/compressor 100 and,when the frequency decreases, the control system should decrease thestroke/capacity. In the same way as in the first preferred embodiment ofthe present invention, it is possible to operate the cooling system bymeans of a simple “On/Off”-type thermostat, maintaining the same conceptof adjustment of the piston 1 (capacity of the compressor 100) byvarying the frequency.

In this regard, one can observe that the basic concept between the firstpreferred embodiment of the present invention and the second embodimentis similar, that is to say, one can observe the effect of the change inthe load applied to the compressor with respect to the resonancefrequency, and, with this information, alter the piston stroke(compressor capacity).

With the control method in this embodiment, one can proceed inaccordance with the flow diagram shown in FIG. 14 and following thesesteps:

measuring the feed frequency of the linear motor 10, which is thevoltage frequency f_(VP), and then making the compensation of thismeasurement with the value of a reference frequency FR, which is usuallyof 50 or 60 Hz.

In this compensation step, if the voltage frequency f_(VP) is higherthan the reference frequency FR, one should increment the capacity ofthe linear compressor 100. If the voltage frequency f_(VP) is lower thanthe reference frequency FR, one should decrease the capacity of thelinear compressor 100.

To have these methods of the first and second embodiment operating inthe best possible condition for the system, the linear compressor 100has to be designed to operate in resonance when the system is stabilizedand at low capacity (in this condition, the system should operate 80% ofthe time). In this way, when a greater capacity is necessary, thealgorithm will increase the capacity of the linear compressor 100.

Another ability which the algorithm should have is the function ofmaximum (rapid) freezing. In a freezer, when this function is active,the linear compressor 100 will function for 24 hours without cycling; insystems with variable capacity, the linear compressor should function atthe maximum capacity, regardless of the load or internal temperature. Inorder to perform this function, the algorithm may measure the cycletime, if this time is longer than a reference (for instance, 2 hours);the algorithm will go to the maximum capacity independently of the phasecondition and will only operate normally again when the system cycles orwhen the 24 hours have passed.

The advantages of the proposed solution are as follows:

-   -   it enables one to apply the linear compressor in simple systems,        equipped with conventional thermostat and to use the advantages        of the variable capacity;    -   it reduces costs of the cooling/refrigerator system 20;    -   it optimizes the functioning of the linear compressor (the        linear compressor always works at the maximum efficiency);    -   the performance of the linear compressor is improved;    -   there is a correction in the pumping capacity of the linear        compressor, adapted to the need of the cooling system 20.

Examples of preferred embodiments having been described, one shouldunderstand that the scope of the present invention embraces otherpossible variations, being limited only by the contents of theaccompanying claims, which include the possible equivalents.

The invention claimed is:
 1. A method of controlling a linear compressor(100) of a cooling system (20), the linear compressor (100) comprising apiston (1) driven by a linear motor (10), the piston (1) having acontrolled voltage (V_(M)), the controlled voltage (V_(M)) having avoltage frequency (f_(VM)) applied to the linear motor (10) and adjustedby a processing unit (22), the controlled voltage (V_(M)) generating afeed current (i_(A)) that circulates in the linear motor (10), whereinthe method comprises the steps of: measuring a feed phase (φ_(C)) of thefeed current (i_(A)) and a dynamic phase (φ_(P)) of the piston (1) ofthe linear compressor (100), measuring a difference between the feedphase (φ_(C)) and the dynamic phase (φ_(P)) and establishing a measuredphase (φ_(PC)), dynamically adjusting a range of displacement of saidpiston as a function of a variation in demand of the linear compressor(100) of the cooling system, so that the linear compressor will be keptin resonance throughout the variation in demand so that a value of themeasured phase (φ_(PC)) will be null, wherein said variation in demandis derived from changes in said measured phase (φ_(PC)) withoutreference to an external variable.
 2. A method according to claim 1,wherein, after the step of establishing the measured phase (φ_(PC)),further comprising a step of increasing the range of piston (1)displacement when the value of the measured phase (φ_(PC)) is positiveor a step of decreasing the range of piston (1) displacement when thevalue of the measured phase (φ_(PC)) is negative.
 3. A method accordingto claim 2, wherein said method further comprises, after the step ofincreasing or decreasing the range of piston (1) displacement, waitingfor the passage of a stabilization time.
 4. A method according to claim3, wherein, after the passage of the stabilization time, said methodfurther comprises a new measurement of the difference between the feedphase (φ_(C)) and the dynamic phase (φ_(P)).